Nowadays, electric and electronic equipment which are intended to be marketed must meet some standards on electromagnetic interference (EMI) that are promulgated by FCC or CISPR. Usually EMI can be classified into differential-mode (DM) noise and common-mode (CM) noise. The differential-mode current flows between the Line and the Neutral, and the DM noise level is determined by the ripple current of input terminal of the power supply. The common-mode current flows between the power line and earth ground, and CM noise is caused by charging and discharging the parasitic capacitance between hot-voltage points, where the voltage jumps rapidly in the circuit within the EMI test frequency range, and earth ground.
Usually, the common-mode current is not determined by only one capacitance. FIG. 1 shows a power supply with a flyback converter topology. The flyback converter 100 of FIG. 1 includes a bridge rectifier 110, a filtering capacitor 112 connected in parallel with the bridge rectifier 110, a transformer T100 having a primary winding N100 and a secondary winding S100, a switch 114 connected to the primary winding N100, a rectifier diode 116 connected to the secondary winding S100, and an output capacitor 118. The flyback converter 100 is configured to power a load 120. Also, a standard Line Impedance Stabilizing Network (LISN) 130 is provided and connected to the input terminal of the flyback converter 100 and is configured to provide the constant impedance for the converter to measure the EMI noise, and also to provide isolation for the equipment under test (EUT) from the ambient noise on the power lines. The ON/OFF operations of the switch 114 produce high dv/dt on the point P in the circuit, and the point P is a hot-voltage point. Point B and point S have the same polarity. The common-mode current 3 goes to earth ground via a primary side capacitance CP, a transformer parasitic capacitance CPS and a secondary side capacitance CS—G, wherein CP and CS—G are the capacitance between the power supply and earth-ground. As shown in FIG. 1, the equivalent common-mode capacitance of the power supply is the result of the secondary side capacitance CS—G in series with the transformer parasitic capacitance CPS, and then in parallel with the primary side capacitance Cp. There have been numerous techniques to reduce the primary side capacitance CP, and the capacitance of the secondary side capacitance CS—G in series with the transformer parasitic capacitance CPS is determined by the transformer parasitic capacitance CPS since the secondary side capacitance CS—G is much larger than the transformer parasitic capacitance CPS. Therefore, the equivalent common-mode capacitance is finally dominated by the value of the transformer parasitic capacitance CPS. The earlier we know the capacitance of the transformer parasitic capacitance CPS, the better in designing the EMI filter circuit. Hence, if we can obtain an accurate value of CPS, the quality control in mass production will be improved.
Usually there are two methods to get parasitic capacitance CPS, one is calculation, and the other is test. It is known that models can be built based on the transformer structure, and the parasitic capacitance between the primary and secondary side of the transformer can be calculated in static electric field. But the results deviate from the fact, because when the switching mode power supply is operating, the voltage along the winding varies, thus the voltage of each turn of the transformer winding are different. However, the calculation in the static electric field doesn't consider about it. So there is no simple method to calculate this equivalent common-mode capacitance accurately in engineering application.
FIG. 2A and FIG. 2B show the conventional test apparatus for calculating the common-mode capacitance, in which FIG. 2A shows the internal configuration of the test apparatus and FIG. 2B shows the testing configuration of the test apparatus with the transformer. The test apparatus 7 is typically an impedance analyzer or LCR meter. The calculation carried out by the conventional test apparatus 7 is incorrect because either impedance analyzer or LCR meter is one-port network with only two terminals 8 and 9. The inner voltage source 10 of the test apparatus 7 is connected to the equipment under test 11 via terminal 8 and 9. After the current response 12 and voltage response 13 are obtained, the impedance characteristic of equipment under test 11 can be calculated. It is obvious that in the one-port network 7, the voltage source 10 and the responses 12, 13 are obtained from the same terminal 8 and 9. In FIG. 2B, LCR meter or impedance analyzer can measure the parasitic capacitance 14 between the primary side winding 15 and the secondary side winding 16 accurately. But the common-mode current between the primary side and the secondary side is the displacement current which is related with the voltage drop between the primary and secondary winding, as shown in FIG. 3. FIG. 3A shows the transformer structure without shielding and FIG. 3B shows the transformer structure with shielding. Suppose the turns of the primary winding 15 are larger than that of the secondary winding 16, and the bottom turn 17 is usually connected to point B in FIG. 1, where the voltage is almost stable in EMI test frequency range, and we denote it as a quiet point. The top turn 18 is usually connected to hot-voltage point P in FIG. 1, where the voltage jumps rapidly. And turn 18, 20 have the same polarity. It is obvious that the voltage difference between turn 17 and 19 is almost zero, thus there is almost no common-mode displacement current. The voltage difference between turn 18 and 20 is the largest, thus there is the largest common-mode displacement current between them. The capacitance 14 in FIG. 2B can not disclose such phenomena, so it is not the capacitance we wanted. In other words, the test method as shown in FIG. 2 doesn't consider the voltage variations along the windings 15 or 16 from the top winding portion e.g. turn 18 or 20 toward the bottom winding portion e.g. turn 17 or 19. And since there is no voltage difference between the windings 17 and 19, there is no voltage difference between windings 15 and 16. If there is no voltage difference between the winding 15 and the winding 16, the displacement current is zero although capacitance 14 exists, so capacitance 14 has no contribution to common-mode current. Obviously the capacitance 14 is not the key common-mode capacitance CPS mentioned above. If there is a shielding layer 21 between the primary side winding 15 and the secondary side winding 16 in the transformer, one-port network can only measure the parasitic capacitance 22 between shielding layer 21 and secondary side winding 16. This capacitance 22 can not represent the equivalent CM capacitance CPS either.
Based on the foregoing descriptions, it is intended by the applicant of the invention to disclose a test apparatus of measuring equivalent parameters. The proposed test apparatus doesn't depend on completed product test, and can get the accurate parasitic parameters in the early design cycle, thereby helping in designing EMI filter in a switching mode power supply. It can be also used to control the transformer quality during the mass production, reduce the control cost effectively. Therefore, it is an imminent tendency to develop a test apparatus and method applied to the power converter such that the aforesaid deficiencies and disadvantages can be addressed.